Since deterioration of a plasma electrode and a nozzle directly degrades quality of worked products, and increases running cost on replacing the electrode and the nozzle in plasma arc thermal cutting of a metal plate material (hereinafter, called a work), various research and development conventionally continue to be performed regarding the life of them.
First, to facilitate understanding of the present invention, as a prior art of the plasma cutting machine, the most basic cutting machine using oxygen as a plasma gas and a control method thereof will be explained with reference to FIG. 4A to FIG. 4F which show a general constitution of the plasma cutting machine and a plasma arc starting method, and FIG. 5 which is a time chart of an operation sequence showing an arc starting control method of this plasma cutting machine. In FIG. 4A, the plasma cutting machine includes a plasma power supply (for example, a constant current power supply) 8, a relay box (for example, high frequency generator) 9 which is connected to the constant current power supply 8 by a power supply cable 51, and a plasma torch 1 which is connected to the high frequency generator 9 by a torch cable 52. A work 11, which is connected to the constant current power supply 8 by a base material cable 53 provided in parallel with the power supply cable 51 and the torch cable 52, is cut by plasma arc of the plasma torch 1.
In FIG. 4B and FIG. 5, when a start signal ST is inputted into the plasma cutting machine, the constant current power supply 8 is actuated, then a switch (electromagnetic switch) 14 is closed, and direct-current voltage is applied, so that an electrode 1a inside the plasma torch 1 becomes minus, and a nozzle 1b and the work 11 become plus. At the same time, a stop valve 15 is opened, and an oxygen gas as pre-flow is supplied into the plasma torch 1. The pre-flow is provided to replace air inside a gas conduit line 4 with oxygen completely, and to obtain sufficient time until a gas flow rate is stabilized.
After the above-described pre-flow, in FIG. 4C, when the high-frequency generator 9 is actuated and high-frequency high voltage is applied between the electrode 1a and the nozzle 1b, a spark discharge occurs between the electrode 1a and the nozzle 1b. As shown in FIG. 4D, with this spark discharge as a seed, a pilot arc 16 is formed between the electrode 1a and the nozzle 1b, a pilot current Ip flows through a circuit from the constant current power supply 8 via a resistance 12, the switch 14, the nozzle 1b, the pilot arc 16 and the electrode 1a to return to the constant current power supply 8. In this situation, in brief, the constant current power supply 8 is in a state in which it outputs the maximum output power, namely, it functions as substantially the constant current power supply, and therefore the pilot current Ip is given a drooping characteristic by the resistance 12, and is stabilized in a state in which a power supply characteristic and arc voltage are balanced.
As shown in FIG. 4E, when electrical continuity is secured between the electrode 1a and the nozzle 1b with the pilot arc 16 (see FIG. 4D) as guidance, part of a pilot current Ip becomes a main current Im and flows into the work 11 to form a main arc 13. This is detected with a current detector (not shown), and as shown in FIG. 4F, the switch 14 connected to the nozzle 1b is detached, whereby the circuit is only for the main arc 13, and only the main current Im passes through it. A constant current control is performed while comparing an output value of the current detector and the set value to keep a cutting current value previously set (main current Im), and cutting work of the work 11 is carried out. Thereafter, when cutting is finished, a stop signal SP is inputted into the power supply, the output of the power supply is stopped, supply of electric power to the main arc 13 is stopped, and the main arc 13 disappears.
As described above, according to the prior art, the resistance and the switch (electromagnetic switch) are placed in series in the pilot circuit, and after the pilot arc occurs, the main arc is detected by means of the main arc detecting means. Then, according to the detection signal, the switch is opened to interrupt the pilot arc, and the main arc is ignited, which is an art generally adopted in the plasma cutting machines. As for general required time in each process step, the pre-flow in FIG. 4B requires about 2 sec, the time period from the high-frequency high voltage application to the occurrence of spark discharge in FIG. 4C requires about 6 μsec, and the main arc transfer in FIG. 4E requires about 20 to 30 sec.
As a technical challenge in the plasma cutting machines so far, extension of the lives of consumable components is first cited, and a number of inventions are made therefor. As the first prior art, Japanese Patent Laid-open No. 5-104251 is cited. This Laid-open Patent reports that the effect of reduction in electrode consumption is obtained by the art of switching the plasma gas from low gas pressure to high gas pressure, or switching it from a small flow rate to a large flow rate, directly after arc ignition, as the manner of feeding the plasma gas which is supplied to the plasma torch.
Japanese Patent Laid-open No. 3-258464 as the second prior art discloses the art of supplying a non-oxidizing gas to the plasma torch as a plasma gas at the time of starting arc, before or directly after starting arc, and switching the plasma gas to an oxidizing gas after arc ignition. It describes that according to the art of switching the kinds of gas, electrode consumption can be reduced, and the life of the electrode can be extended.
Japanese Patent Laid-open No. 6-15457 as the third prior art describes the art of adopting a transistor instead of an electromagnetic switch as a switch when the pilot arc is interrupted by opening the switch according to the ignition detection signal of the main arc to improve transferability from the pilot arc to the main arc. This art relates to a secondary side chopper control of a pilot current, and the transistor is made to function as a chopper control element, not as a simple switch. The circuit of the main arc outputs full power during occurrence of the pilot arc, and large voltage between the electrode and the base material, or between the nozzle and the base material, which is necessary for the transfer, can be taken sufficiently, therefore making it possible to prevent transfer error and transfer delay to the main arc and provide a favorable power supply device.
The arts related to only the extension of life of electrodes exist conventionally as seen in the aforementioned first and the second prior arts. However, regarding the plasma gas switching of these prior arts, it is recently found out that if the gas is made at a low flow rate or at low pressure at the time of starting, transferability from pilot arc to main arc becomes worse, and a problem of increasing damage of the nozzle occurs. It is found out that transferability to main arc becomes worse with a nitrogen gas or a gas including a lot of nitrogen than with oxygen, and damage is larger to the nozzle. Namely, switching of the gas significantly contributes to an increase in the life of the electrode, but it is found out that it provides no improvement or even an adverse effect concerning the life of the nozzle. Accordingly, even if the life of the electrode is extended, the life of the nozzle terminates before termination of the life of the electrode, and therefore replacement interval of these consumable components does not become long as expected. The fact is that even if this art is adopted and the life the electrode is extended (from about 200 times to about 600 times in the number of arc ignition times), the life of the nozzle is not improved with the number of arc ignition times being about 150 times to 200 times at most.
As the factors responsible for damage to the nozzle, the following two are cited. One is the case caused by a so-called external factor in which molten metal (spatter) that blows toward the nozzle during a piercing process (punching process) adheres to the nozzle, and thereby the nozzle is damaged. The other one is the damage which is caused as a result that an electric current flows into the nozzle by the pilot arc and the outlet port portion of the nozzle is melted by the time of transfer from the pilot arc to the main arc. As the remedial steps for the factors having adverse effects on the life of the nozzle, there is a method of protecting the nozzle from the spatter by providing a shield cap outside the nozzle as to the former external damage, and this method is adopted by most of the plasma torches at present. However, as to the latter damage caused by the pilot arc, clear disclosure of the method of reducing it is not found in the prior arts yet. In short, the art of extending the lives of the electrode and the nozzle at the same time and bring about sufficient practical effects does not exist in the prior art.
As for the life of the nozzle, so long as the life of the electrode does not terminate, the portion in the vicinity of the outlet port of the nozzle is melted by the pilot arc which frequently occurs between the electrode and the nozzle until the transfer to the main arc from the pilot arc is performed, whereby the damage is gradually expanded, normally. Then, the sharpness of the arc is reduced, and at the stage in which the cutting work accuracy is below a predetermined value, it is determined that the life terminates.
Meanwhile, it is known that the temperature of the electrode surface rises up to the high temperature of about 3000° C. at the time of starting arc, and it is instantly consumed in such a manner as the electrode surface is peeled off by the thermal shock at this time. The life of the nozzle is influenced by the life of the electrode. For example, when the electrode reaches some stage of its life, it is abruptly damaged and broken for the aforementioned reason, and at this time, the arc between the electrode and the work in the nozzle, which is thermally cutting the work, is stopped. Then, in place of it, arc between the nozzle and the electrode occurs, and it instantly (on the same principle as that the arc melts the work) melts a portion in the vicinity of the outlet port of the nozzle.
As described above, when an instant damage to the electrode occurs, a so-called accompanying nozzle damage is caused by this. Consequently, the fact is that even when the nozzle still has sufficient sharpness as a nozzle and does not reach the end of the life directly before the damage, it is instantly brought into the state in which it cannot be used continuously. For the above-described reason, from the fact that how many life extension measures are taken for only the life of the nozzle, the life of the nozzle is determined according to the life of the electrode, much attention is paid to the development of the art of extending only the life of the electrodes. Accordingly, it can be said that the idea does not reach the consideration from the viewpoint of relating the nozzle life extension art to the electrode life extension art.
As for the electrodes and the nozzles, the replacement frequencies due to the life span thereof have a tremendous influence not only on the consumption cost accompanying the replacement of the electrode and the nozzle, but also on the reduction in machine availability (productivity reduction), which becomes the problem. To solve the problem, it is ideal to extend the lives of the electrode and the nozzle as long as possible, respectively, and to replace them as a set at the same time (give them the same life span). However, the reality is that the life of the electrode and the life of the nozzle are not the same, and the nozzle life is influenced by an abrupt damage to the electrode as described above, and therefore, their lives cannot help being set to be shorter with an allowance being given.
The adoption of the transistor in the arc ignition art described in the above-described third prior art does not intend extension of the life of the nozzle as described above. In addition, the transistor interposed in the pilot line is used not only as a switch, but a chopper element for adjusting the pilot current. Consequently, a constant current control circuit to control the transistor becomes necessary apart from the constant current control circuit of the main arc current, and the power supply is complicated and the cost is increased.